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<meta charset="UTF-8">

<title> 左定宽，右主动 </title>

<style type="text/css">

*{margin:0; padding: 0;}

.box { float:left;height:300px; width: 100%;background-color:#999;}

.box .cont {margin-left:210px;background-color:#ccc;height:100%;width:100%;}

.asider {float: left; width: 200px;height:100%; margin-left: -100%; background-color:#ccc;}

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<body >
<div style='width:600px;background-color:#3c;'>
<div class="box">

    <div class="cont"><div style="width:100%;">ssss</div></div>

</div>

<div class="asider">左定宽，右主动<br />左定宽，右主动<br />左定宽，右主动<br />左定宽，右主动<br />左定宽，右主动<br />左

定宽，右主动<br />左定宽，右主动<br /></div>
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</body>

</html>